Solar operated systems now in operation for powering telecommunication projects require from 100 to 3000 watts on a continuous basis, day and night. By reason of the dependence of such systems on sunlight, over-design of the systems are required in order to meet contingent operating and environmental conditions. For example, an array of photovoltaic sources is typically sized eight or more times larger than the peak load; and a ten day battery back-up reserve is ofentimes provided.
In an effort to down-size both the photovoltaic array and the back-up battery, hybrid systems are in use utilizing a fueled prime-mover driving a generator that charges the batteries. One such hybrid system employs a Rankine cycle organic fluid turbogenerator (hereinafter referred to as a power plant of the type described) manufactured by Ormat Turbines Ltd. and described in U.S. Pat. No. 3,409,782.
Such a hybrid system is described in "Closed Cycle Vapor Turbogenerator - A Reliable Remote Prime Power Source" by N.S. Christopher and J. Gropper, presented at the International Telecommunications Energy Conference in Washington, D.C. October 1982 and printed in pages 443-449, which is hereby incorporated by reference.
A power plant of the type described has a Rankine cycle turbogenerator utilizing an organic fluid, such as a chlorobenzene or Freon, as the operating fluid; and the turbogenerator is hermetically sealed in a cannister. Only the turbine and the generator of the turbogenerator move; and the shaft carrying these components is mounted on bearings lubricated with the operating fluid thus eliminating metal-to-metal wear and permitting the turbogenerator to run continuously for long periods of time without overhaul and with little maintenance. An external, fuel-fired burner heats an evaporator which vaporized the liquid organic fluid and supplies hot vapor to the turbine. The vapor expands in the turbine which drives the generator producing power. Exhaust vapor from the turbine is condensed in an air-cooled condenser; and the resultant liquified organic fluid is returned to the evaporator. Some of the condensate, however, is returned to the evaporator through the bearings of the turbogenerator as described previously. A single on/off fuel supply to the burner is used for controlling the voltage of the system: the burner is turned-on whenever the voltage of the system drops below a first threshold and is turned-off whenever the voltage of the system rises above a second threshold. This arrangement provides a particularly simple and reliable control for the turbogenerator. Because of the high starting and running reliability of the system of the type described, the hybrid power plant described above has a battery capacity that is only 60% as large as the capacity of a power plant that relies upon photovoltaic cells only.
Another advantage of the hybrid power plant that arises because of the reduction in battery capacity is the reduction in self-discharge losses in the batteries. Consequently the trickle charge required to maintain the battery is also smaller than a power plant relying only on photovoltaic cells. The hybrid power plant also enjoys a significant cost advantage when the static load increases into the multi-hundred watt range.
A hybrid power plant of the type described is thus very advantageous in terms of cost and reliability; and it is an object of the present invention to provide a new improved hybrid power plant of the type described which is even more efficient and cost effective.